Morphology control of polybenzimidazoles was examined by reaction-induced phase separation during polymerization. Polymerizations of 3,3'-diaminobenzidine with terephthalic acid or diphenyl terephthalate were carried out in poor solvents. The morphology of the precipitated poly[2,2'-(1,4-phenylene)-5,5'-bibenzimidazole] (PpBBI) was significantly influenced by the polymerization conditions, and the aggregates of nano-scale PpBBl fibers were obtained by the polymerization at a concentration of 3-5% and 320-350 degrees C in dibenzyltoluene. The average diameter of the fibers was ca. 50 mn and inherent viscosities of the precipitates were 0.35-0.58 dL g(-1). They possessed high crystallinity and thermal stability. The oligomers were precipitated first by the reaction-induced crystallization to form the highly crystalline lath-like crystals at an initial stage of polymerization. Then the lath-like crystals were split into disentangled aggregates of fine fibers with maintaining the high crystallinity. The polymerization mainly proceeded when the oligomers were registered into the crystals. The obtained aggregates of nano-scale fibers could be recognized as nonwoven fabrics. Morphology control of poly [2,2'-(1,3-phenylene)-5,5'-bibenzimidazole] was also examined and particles were mainly formed. (c) 2008 Elsevier Ltd. All rights reserved.